As a transparent wiring board used, for example, for a touch panel, one using a glass substrate is hitherto known. This transparent wiring board is formed such that an ITO (Indium Tin Oxide) film is formed on a glass substrate and then processed by etching, to form a transparent conductive film (transparent wiring pattern) having a desired pattern. Since the glass substrate is used, this transparent wiring board has a problem of being heavy and fragile, and further being high-priced.
Moreover, there has recently been a demand for a light-weight transparent wiring board having impact resistance due to application to electronic paper and the like.
Hence there is known a transparent wiring board using, as a substrate, a transparent film made of polyethylene terephthalate (PET) or polyethylene naphthalate (PEN).
The transparent wiring board is produced in the following manner. First, an ITO film is formed by sputtering on a transparent film made of PET or PEN. Subsequently, an exposure process, a development process and the like are performed to form a resist layer having a predetermined pattern on the ITO film. The ITO film is then etched with this resist layer used as a mask, to form a transparent conductive film having a desired pattern on the transparent film.
However, the above transparent wiring board has a problem of having heat resistance not sufficient to be used as a printed wiring board where components are mounted.
Normal PET has a heat-resistant temperature in the order of 130° C., and even heat-resistant PET has a heat-resistant temperature of about 150° C. For this reason, a transparent wiring board using PET is not applicable to a product (touch panel, electronic paper, LED illumination, EL illumination, etc.) required to be subjected to heat treatment at a high temperature such as a reflow process or an ACF (Anisotropic Conductive Film) connecting process.
Further, the ITO film formed on the transparent PET film has not succeeded in acquiring sufficient characteristics, in terms of flexibility and adhesion to the transparent film, to be used as a flexible printed wiring board required to have bending resistance (see Non-patent Document 1).
On the other hand, PEN has a relatively high heat-resistant temperature (about 180° C.), and can thus be subjected to a process which is a low-temperature reflow process using special solder or a low-temperature ACF connection process. However, PEN and the special solder are high-priced and special manufacturing facilities are required, thereby causing a problem of increased production cost.
Moreover, in the case of performing an etching process for forming a transparent wiring pattern, material use efficiency is low since lots of materials are disposed, and further, a large amount of carbon dioxide (CO2) is generated. The process is thus required to be changed to an environment-friendly process.